feat(action): built-in remote Action opens the git remote on ctrl+g (#65)

[hammadmajid]

What changed

• git.WebURL rewrites a remote URL to the repo's https home page (scp-style, ssh:// with/without port, git://, https with/without .git, GitLab subgroups, credentials stripped; local paths are an error).
• git.RemoteURL(ctx, dir) picks origin, else the first remote, and returns git.ErrNotRepo / git.ErrNoRemote.
• Go-side Action seam: {remote} in an Action's Run expands (shell-quoted) to that URL. action.Command now returns (*exec.Cmd, error) and fails when {remote} can't be resolved, so Start/Run return the error and the Picker shows remote: no git remote on its footer and stays open. Nothing is looked up when Run has no {remote}.
• Built-in remote Action: ctrl+g, <opener> {remote}, detached. [actions.remote] key and run override it as for any built-in.
• README documents the Action and {remote}.

Verification

gofmt clean, go vet, golangci-lint run ./... (0 issues), go test ./... (only the known local fish TestScriptExecution failures), binary builds, no file over 300 lines. Table tests for the URL rewrites; real-repo tests for origin/first-remote/no-remote/not-a-repo; Picker footer test.

Closes #65